nimbostratus

As the name says nimbostratus bring drops. They are mid-level clouds that develop above 2000 m, they are dark grey and are shapeless, in fact they cover the sky with a veil or stratus that blocks the sun light. In these photos nimbostratus are accompanied by altostratus and let the sun shining through them, though the drops is approaching quickly.

photo -> nature -> clouds -> medium level clouds -> nimbostratus